As illustrated in FIG. 4, an electrically driven type of power steering apparatus is provided with an input shaft 12, of which one end is fixed to a steering wheel 10 and other end is connected to an output shaft 14 via a torque detecting device 22. The other end of the output shaft 14 is engaged with a rack shaft 16 of a rack and pinion mechanism. Both ends of the rack shaft 16 in a lateral direction are connected to mechanisms for controlling directions of vehicle wheels 20. The input shaft 12 is rotated in response to operation of the steering wheel 10, thereby leading to rotation of the output shaft 14. The rack shaft 16 is slidably moved in the vehicle lateral direction in response to the rotation of the output shaft 14, thereby capable of controlling the directions of the vehicle wheels 20 in response to the slidable movement of the rack shaft 16.
Frictional force is generated between the vehicle wheels 20 and a road surface. In such a case, a relatively great steering force is required to properly control the directions of the vehicle wheels 20 only with operational force applied to the steering wheel 10. Therefore, the electrically-driven power steering apparatus illustrated in FIG. 4 has been currently developed. The electrically-driven power steering apparatus is provided with a motor 18 for slidably moving the rack shaft 16 in the vehicle lateral direction. The torque applied to the input shaft 12, i.e. the operational force applied to the steering wheel 10, is detected by the torque detecting device 22. The motor 18 is then driven so as to generate force for assisting the steering operation of the steering wheel 10, thereby capable of generating an appropriate steering force for controlling the direction of the vehicle wheels 20. A controller 24 is provided between the torque detecting device 22 and the motor 18.
The torque detecting device 22 for this type of electrically-driven power steering apparatus is generally provided with a resolver capable of detecting a rotational angle with higher detecting precision compared with other rotational angle detectors. Therefore, the torque detecting device provided with such a resolver can detect torque with higher detecting precision. The aforementioned torque detecting device 22 is provided with a torsion bar spring (i.e. an elastic member), of which one end is connected to the input shaft and other end is connected to the output shaft. A first resolver is disposed adjacent to a joint portion of the torsion bar spring and the input shaft and a second resolver is disposed adjacent to a joint portion of the torsion bar spring and the output shaft. The first resolver detects a rotational angle θ1 of an end portion of the torsion bar spring at a side of the input shaft and the second resolver detects a rotational angle θ2 of an end portion of the torsion bar spring at a side of the output shaft. Therefore, the torque applied to the torsion bar spring is calculated in accordance with an equation; G×(θ1−θ2) (G: coefficient). The torque applied to the torsion bar spring is substantially equal to the torque applied to the input shaft 12 or the output shaft 14.
The resolver is a rotational angle detecting unit with a motor structure and is provided with an excitation winding coil and an output winding coil. A magnetic field is created around a rotary shaft in response to applying excitation current to the excitation winding coil. The magnetic field created around the rotary shaft is detected by the output winding coil set around the rotary shaft The output voltage form the output winding coil is increased or decreased corresponding to the rotational angle of the rotary shaft. Therefore, the rotational angle of the rotary shaft can be detected by monitoring the increase or decrease of the output voltage from the output winding coil,
Recent demands have lead to improving a detecting precision for detecting the torque applied to the rotary shaft by the torque detecting device. For example, according to the electrically-driven power steering apparatus illustrated in FIG. 4, the motor 18 is required to be controlled to precisely generate torque for assisting the operational force for operating the steering wheel 10 so as to improve the total steering performance. In this case, the operational force applied to the steering wheel 10, i.e. the torque applied to the input shaft 12, is required to be detected with higher detecting precision. According to the known torque detecting device, the torque can be detected with a relatively high detecting precision by employing a resolver. However, a recently demanded detecting precision can not be achieved.
The present invention therefore seeks to provide an improved torque detecting device capable of detecting the torque applied to the rotary shaft with a higher detecting precision.